1. Technical Field
The present invention relates to a surface mounted tuning-fork type piezoelectric unit. In particular the invention relates to a tuning-fork type piezoelectric element, and a metal bump which connects to a container main body that accommodates the resonator.
2. Background of the Invention
A tuning-fork type piezoelectric unit is widely used as a clock frequency source of electronic equipment. In recent years, due to the miniaturization of the electronic equipment in which these are built-in, the size of the tuning-fork type piezoelectric unit is also required to be miniaturized and thinned.
Prior Art
FIG. 5A and FIG. 5B are drawings for describing a conventional tuning-fork type piezoelectric unit, wherein FIG. 5A is a perspective view of a tuning-fork type piezoelectric unit showing the interior opened with a part of the container main body removed, and FIG. 5B is a cross-sectional side view thereof on V-V (FIG. 5A). FIG. 6A and FIG. 6B are drawings for describing a tuning-fork type piezoelectric element, wherein FIG. 6A is a front view of the tuning-fork type piezoelectric element, and FIG. 6B is a schematic plan view showing the electrical connections of the tuning-fork type piezoelectric element.
The tuning-fork type piezoelectric unit, as shown in FIG. 5A and FIG. 5B comprises a container main body 1 with a tuning-fork type piezoelectric element 7 accommodated inside thereof, and is covered with a cover 2. The tuning-fork type piezoelectric element 7, is made for example from crystal, and as shown in FIG. 6A, a pair of tuning-fork arms 9 extend out from a tuning-fork a base 10. Moreover, it has excitation electrodes 9a on each of the four faces of the pair of tuning-fork arms 9, and lead out electrodes 10a extend out on one principal plane of the tuning-fork base 10 in a wiring pattern (not shown in the figure). The tuning-fork arms 9, as shown in FIG. 6B, are connected with common potentials between the two principal planes and the two side faces of the tuning-fork arms 9. Moreover, they are connected to the pair of lead out electrodes 10a provided on the one principal plane of the tuning-fork base 10.
The container main body 1 comprises a laminated ceramic substrate with a concave cross-section which has on an inside wall on one end side, for example a divided step 3. In this conventional example, it comprises a three layer structure comprising ceramic plates (1a, 1b, and 1c) in order from the open face. On an upper surface of this inside wall step 3 positioned on the container main body 1, electrode pads 4 are formed. The electrode pads 4 are formed with for example tungsten (W) as the ground electrode, and for example nickel (Ni) film as the intermediate material, and a conductive layer of gold (Au) film.
The ground electrode (W) is formed for example by printing and baking, and the intermediate material (Ni) and the conductive layer (Au) are formed by electroplating. Furthermore, on the upper surface of the electrode pad 4 there is formed a metal bump 6 comprising gold (Au). The metal bump 6 is formed by the aforementioned printing and baking, or by electroplating or the like. As required, the metal bump 6 may be formed by a print bump or a plating bump.
Furthermore, both end sides in the one principal plane of the tuning-fork base 10 with the lead out electrodes 10a extending from the excitation electrodes 9a are bonded to the metal bump 6 by a conductive adhesive 8. The conductive adhesive 8 is for example a heat hardening type, and after applying onto the metal bump 6, the tuning-fork base 10 is positioned. Then it is pressured (compressed) from above the tuning-fork base 10, and heat hardened. As a result, the lead out electrodes 10a extending from the excitation electrodes 9a of the pair of tuning-fork arms 9 are connected electrically to the mounting terminals 5 provided on the bottom face of the container main body 1, through the metal bump 6, the electrode pads 4, and a wiring path (not shown in the figures). The cover 2 is joined to the opening end face of the container main body 1 by a seam weld or the like, and the tuning-fork type piezoelectric piece 7 is hermetically sealed. Refer to Japanese Unexamined Patent Publication No. 2004-312057).
Problems With the Prior Art
However, in the tuning fork type piezoelectric unit of the above configuration, the metal bump 6 is formed in a flat shape, and hence at both end sides in the one principal plane of the tuning-fork base 10 with the extended lead out electrodes 10a, this is completely opposed and closely bonded. Consequently, the adhesive strength of the tuning-fork type piezoelectric piece 7 depends on the contact surface area which is closely contacted with the metal bump 6. On the other hand, regarding the tuning-fork type piezoelectric piece 7, in order to increase the adhesive strength of the conductive adhesive 8 with respect to external impact, and the electrical conductivity, it is desired to increase the contact surface area (facing surface area) with the metal bump 6. In this case, oscillation leakage from the tuning-fork base 10 of the tuning fork vibration due to the pair of tuning forks 9 becomes large in proportion to the contact surface area with the metal bump 6. Furthermore, the greater the oscillation leakage, the lower the oscillation efficiency of the tuning fork oscillation so that vibration characteristics in a stationary condition (a condition with no impact from the outside) where the crystal impedance (CI) is high are deteriorated. Moreover, even if the vibration characteristics in the stationary condition are maintained, with a larger the contact surface area, the amount of the conductive adhesive 8 between the tuning-fork base 10 and the metal bump 6 also increases, so that before and after the drop impact test, the change in the vibration frequency becomes great. That is to say, regarding the conductive adhesive 8, the condition changes due to a drop impact, and the retention state with respect to the tuning-fork base 10 changes. In this case, the bond strength is reduced due to the impact, so that the restraining force of the tuning-fork base 10 also weakens, and the vibration frequency is reduced. Furthermore, with a larger amount of the conductive adhesive 8, the retention condition also changes greatly, and hence the frequency change amount before and after a drop impact is also great. Due to these matters, the contact surface area where the flat shape metal bump 6 is closely contacted by means of the conductive adhesive 8 is severely limited to within a constant value. However, if the size of the tuning-fork type piezoelectric piece 7 becomes small, for example, if the length of the tuning-fork base 10 becomes somewhat less than 0.5 mm, with the thickness of the tuning-fork type piezoelectric piece 7 at 0.12 mm, the overall length 2.3 mm, and the width 0.5 mm, the positioning of the tuning-fork base 10 with respect to the metal bump 6 becomes difficult. Consequently, in the tuning-fork type piezoelectric unit with the metal bump 6 as a flat shape, then as well as the problem of maintaining the vibration characteristics in the stationary condition with the bond strength and the electrical conductivity maintained, there is a limit to suppressing the frequency change for before and after a drop impact.
Object of the Invention
An object of the present invention is to provide a tuning-fork type piezoelectric unit in which as well as maintaining the vibration characteristics in the stationary condition when miniaturized, the bond strength is maintained, and also the frequency change for before and after a drop impact is suppressed.